feat(email): 初始化自建邮箱 MVP

- 新增 Go + SQLite 后端，支持登录、域名、邮箱、别名、联系人、规则、统计与邮件收发。
- 新增 Webmail 前端，支持多邮箱切换、邮件列表/阅读/写信、附件、搜索、主题切换与个人中心。
- 新增 Docker Compose 部署骨架，补充 Postfix、Dovecot、OpenDKIM、Nginx 配置与部署说明。

apps/web/src/pages/login.tsx

@@ -0,0 +1,42 @@
+import { Navigate } from "react-router-dom"
+import { useMutation, useQueryClient } from "@tanstack/react-query"
+import { api } from "@/lib/api"
+import { useMe } from "@/hooks/use-me"
+import { Button } from "@/components/ui/button"
+import { Input } from "@/components/ui/input"
+import { Label } from "@/components/ui/label"
+import { useToast } from "@/hooks/use-toast"
+
+export function LoginPage() {
+  const me = useMe()
+  const qc = useQueryClient()
+  const { toast } = useToast()
+  const login = useMutation({
+    mutationFn: (form: FormData) => api.login(String(form.get("email")), String(form.get("password"))),
+    onSuccess: async () => { await qc.invalidateQueries({ queryKey: ["me"] }) },
+    onError: (e) => toast({ title: "登录失败", description: e.message }),
+  })
+  if (me.data?.user) return <Navigate to="/mail" replace />
+  return (
+    <div className="grid min-h-screen place-items-center bg-background px-4">
+      <div className="w-full max-w-[360px]">
+        <div className="mb-10 text-center">
+          <h1 className="text-3xl font-bold tracking-tight">LanQin Email</h1>
+        </div>
+        <form className="space-y-5" onSubmit={(e) => { e.preventDefault(); login.mutate(new FormData(e.currentTarget)) }}>
+          <div className="space-y-2">
+            <Label htmlFor="email">邮箱</Label>
+            <Input id="email" name="email" type="email" defaultValue="admin@lanqin.local" required className="h-11 text-base" />
+          </div>
+          <div className="space-y-2">
+            <Label htmlFor="password">密码</Label>
+            <Input id="password" name="password" type="password" defaultValue="ChangeMe123!" required className="h-11 text-base" />
+          </div>
+          <Button className="h-11 w-full text-base" disabled={login.isPending}>
+            {login.isPending ? "登录中..." : "登录"}
+          </Button>
+        </form>
+      </div>
+    </div>
+  )
+}